Gave this problem or lack there of some thought my friend.
Several options come to mind to include what my last resort would be? I personally wouldn’t tap the breech plug.
1. Would see if the last couple of turns of the nipple has no play until it bottoms out. If it has no play “ wiggle “ you do not have a problem.
2. Could try sizing the thread at a hardware store like already mentioned to see if you can match the thread? They’d definitely have to be an understanding business to let you bring the barrel in? Guess you could buy several sizes to bring home I guess? Problem is with that is most places I ever been to don’t sell an individual screw or bolt? If so, you have to be careful in matching up the correct thread pitch? Don’t want to physically force a questionable thread size?
3. Could try teflon tape on the existing nipple to take up the play, if the nipple is solidly bottoming out that is!
Lastly, and is probably what I would do. Like already mentioned, I’d get a quote and have the barrel sent in to have it professionally tapped to the thread size that would be recommended by them? I’d personally ask for 1/4x28 standard if at all possible?
Anyway, if you don’t feel comfortable in doing it yourself my friend, I’d have it professionally done wether it’d be Log Cabin Shop or anyone else of your choosing?
Best of luck to you. Please keep us posted on the outcome?
Respectfully, Cowboy